Advances in Well defined Iron Catalysts for Cross Coupling
Transcript of Advances in Well defined Iron Catalysts for Cross Coupling
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Advances in Well defined Iron Catalysts for Cross Coupling
Fe(I)R'-X
R-R'
Fe(III)RMgX
Michael J. Ingleson
FeIII
X
R'FeIII
R
R'
RMgXMgX2
R-R
•Strengths and Complexities of Fe in Catalysis
•Brief Historical Perspective ‐ From Kochi onwards
Overview
Brief Historical Perspective From Kochi onwards
•Advances in C‐X / C‐M Fe catalysed Cross Coupling (over the last 4‐5 years)
•The Nascent area of direct C‐H Arylation using Fe (last 1‐2 years)y )
• Conclusions
Disclaimer: Its Impossible to be comprehensive in this area! A personal selection.
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Research Driver
2 Supply chain risks
1 Cost + Toxicity
Source: Critical raw materials for the EU (July 2010)
Iron: Has Key Economic Advantages – but more than that:
Ability to access fundamentally different reactivity to Pd/Ni/Cu with cheap ligands
Why Well Defined Iron Catalysis?
Classical Common Non Zero Oxidation StatesIn contrast: Palladium, 0, II, IV
However in reality for Fe :
‐II, 0, I, II, III and IV
All h id i bAll these are oxidation states can be important, is dependent on the catalyst / nucleophile / ligand
Predicting reactivity requires understanding of the dominant mechanism
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Proposed MechanismsFeII/FeIIIFe‐II/Fe0
Or a Fe0/FeII version
FeII/FeIV FeI/FeIII
A Brief Historical Perspective
The birth of ‘Modern’ Cross Coupling:
Corriu and Kumada: 1972
Iron ‐ Tamura and Kochi: 1971
StereospecificCross Coupling
History of Modern Cross Coupling J. Organomet. Chem. 653 (2002) 1 ‐ 303
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Substrate Scope – Alkenyl Derivatives
2nd‐ generation (Broader Substrate scope)
N O
NMP
Key Stabilising ligand
Cahiez, G. et. al. Synthesis 1998, 1199.
Aryl chlorides, triflates and tosylates superior to aryl bromides/iodides (reduction of C‐X) instead
Substrate Scope – Aryl Derivatives
Fürstner, A. et. al. Angew. Chem. Int. Ed. Engl. 2002, 41, 609 and JACS, 2002, 124 13856
However, Aryl Triflates are generally necessary with electron‐rich aryl substrates
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Coupling with Heteroaryl Halides
Fürstner, A. et. al. Angew. Chem. Int. Ed. Engl. 2002, 41, 609 and JACS, 2002, 124 13856
Nickel Vs Iron
Readily Amenable to scale up
> 17g, 84 % Isolated Yield
Component of Liquid Crystals
Fürstner et al. Org Synth 2005, 81, 33‐41
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Possible Mechanisms
Kochi – Active Catalyst ‘a reduced form of soluble Iron’generated in situ and is highly sensitive
Kochi, J. K. J. Organomet. Chem. 2002, 653, 11.Kochi, J. K. et. al. J. Org. Chem. 1976, 41, 502.
Fürstner, et. al. J. Am. Chem. Soc. 2002, 124, 13856.Bogdanovic, et. al. Angew. Chem. Int. Ed. 2000, 39, 4610.
Fe(0)/Fe(II) cycle also proposed
Kochi:
Measurement of methane and ethane during the reduction of Fe(acac)3 with methylmagnesium bromide suggested the value of n = 2 in the following;
Fe(III) + n CH3MgBr Fe(III - n) + X CH4 + Y C2H6
Oxidation State Disparity?
Fürstner:
FeCl2 reacts with 4 equivalent of RMgX to give a new species of the formal composition [Fe(MgX)2] – an inorganic Grignard reagent with Fe at ‐2 oxidation level.
where, n = X + 2Y. Therefore, Fe(III) goes to Fe(I).
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The presence of β‐hydrides is essential
i.
At least Two Different Reactivity Manifolds
ii. Fe(0) is inactive
Fürstner et al., J. AM. CHEM. SOC. 2008, 130, 8773–8787
Fe(II)
Divergent Catalytic Cycles
Fe‐(II)
Aryl, vinyl, allyl and Methyl Grignard Reagents
Without β Hydrides
Fürstner et al., J. AM. CHEM. SOC. 2008, 130, 8773–8787
Grignard Reagents with β Hydrides
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FeX2 or FeX3 with excess MeLi
Organo Ferrate Mechanism
< 20 %
< 20 % Limited to more activated electrophiles
70 %
80 %
A. Fürstner, ACIE, 2006, 45, 440
Low Valent Iron Mechanism
Fe(‐II) is the most active catalyst
racemic
Rates comparable to real systemRadical Intermediates
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Application to Grignards with no β‐hydride
Using the preformed Fe(‐II) catalyst (5 mol%, THF ‐20oC, L = TMEDA)Enables activation of sp3 C‐X and coupling with ArMgBr
Fürstner et al., J. AM. CHEM. SOC. 2008, 130, 8773–8787
1) Whilst achieving sp3 C‐X /ArylMgBr coupling ‐ need simpler, easier to handle catalysts
Inexpensive/Simpler Catalyst Development
Again Need to Overcome:
(i) β‐Hydride elimination ‐ alkene formation, and redcution processes
(ii) Homocoupling (Kharasch 1940s)
2) Furthermore these systems have limited applicability in ArylCl + ArylMgBr coupling
Major by‐products in initial Fe catalysed cross‐coupling of 1o and 2o alkyl substrates with Aryl Grignard reagents
Achieving these goals led to the development of 2 new distinct Catalytic Approaches
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Beyond N‐Me‐pyrrolidone
Solvent Product Yield (%)
C D E F
THF/NMP 25 25 24 26
THF 27 37 20 25
Et2O 60 19 12 12
Et2O (reflux) 69 18 9 8
Hayashi, T. et. al. Org. Lett. 2004, 6, 1297.
Amines, Particularly TMEDA are Key Additives
1st Generation:
Nakamura, E. et. al. J. Am. Chem. Soc. 2004, 126, 3686.
Catalytic FeCl3/Et3N (or DABCO) works well. As does:
Stoichiometric TMEDA as additive8g of product at 90% Isolated Yield
2nd Generation: Catalytic in Amine
Cahiez et al., Angew. Chem. Int. Ed. 2007, 46, 4364Bedford Chem. Commun., 2005, 4161
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Substrate scopeKumada Type Coupling (using TMEDA as ligand)
Negishi type Couplings (aryl Zinc and Alkyl‐Iodides made in‐situ from ArMgBr and ZnI2)
AlkylX Alkyl‐Ar
FeCl3 + TMEDA (5 mole%) ArMgBr (1.2 eq.)
THF 0oC (30min)THF 0oC (30min)
E. And M. Nakamura Org. Lett., Vol. 11, 2009, Pg 4306
Radical Rebound Mechanism (Fürstner/Bedford/Nakamura amongst others)
Mechanism and Role of TMEDA
An FeII/FeIII cycle?
H. Nagashima and M. Nakamura J. AM. CHEM. SOC. 2009, 131, 6078–6079
Outstanding Question: Is TMEDA simply a ligand for Fe or does it attenuate the reactivity of RMgBr as well?
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An Fe Analogue of Suzuki Miyaura Coupling?
Key Catalysts Requirements Defined by Nakamura:
Fe(II) Oxidation state, coordinatively unsaturated, significant spin density on Fe.
M. Nakamura et al., J. AM. CHEM. SOC. 2010, 132, 10674
Beyond TMEDA: (TMEDA)FeCl2 no coupling activity
RLi Base /MgBr2 essential for transmetallation. KF, K3PO4, KOt‐Bu, and Cs2CO3 ineffective
Fe‐OR/ Fe‐F too strong? (diphosphine)FeF2 and (diphosphine)Fe(OEt)2 inactive
Substrate scope – Fe Catalysed. Suzuki Miyaura
Is this really a Fe Suzuki Miyaura Analogue?
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Fe Catalysed sp‐sp2 Sonogashira Coupling?
An Iron analogue of the Sonogashira Reaction was also recently reported
Bolm et al., Angew. Chem. Int. Ed. 2008, 47, 4862
Mechanism? Aryl Iodides only viable
Dmeda ligation required similar to Cu catalysts in Cu catalysed SonogashiraReactions
A Note of Caution!! Highly Active Trace Impurities
Bolm/Buchwald ACIE. 2009, 48, 5586 and Norrby /Bolm et al., ACIE 2009, 48, 5691
If an Fe complex is the active catalyst expect to observe different reactivity patterns, selectivities and/or require different ligands/solvents etc compared to Cu/Pd systems.
Rule of Thumb:
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Bulky diphosphine is essential
A ‘True’ Fe Catalysed Sonogashira Reaction
‘Ferrates’
Radical Mechanism
M. Nakamura Angew. Chem. Int. Ed. 2011, 50, 10973 –10976
Fe Catalysed sp‐sp3 Songashira Coupling
The Fe catalyst is highly chemoselective for sp3C‐X radical FeII/FeIII cycle
Proposed Mechanism
M. Nakamura Angew. Chem. Int. Ed. 2011, 50, 10973 –10976
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FeII/FeIII or FeII/FeIV mechanisms?
Both are feasible ‐ oxidative addition can be a two step radical mediated process
L2FeII
R
RR'X L2FeII
R
RMgX2
RMgX
MgX2
L2FeIII
RR
XR'
L2FeIV
RR
XR'
R'-R
L2FeII
R
X
L2FeII
R
XRMgX
RMgX
R'-R
FeII/FeIIIRadical Rebound
Mechanism
FeII/FeIII/FeIVMechanism
e.g., Why is Fe cross coupling limited to certain nucleophiles / additives?Grignards seems special?
L2FeII
RR
XMX
Assist Transmetallation OR generates a more electron rich metal centre for FeII/FeIII?
There are other outstanding questions:
The Fluoride Effect ‐ A real FeIV intermediate?
Fe Systems discussed so far were poor catalysts for wide scope aryl‐aryl coupling
Nakamura et al., JACS. 2007, 129, 9844, and JACS 2009, 131, 11949–11963
N‐Heterocyclic Carbene Ligand and Fluoride Anion are essentialA range of phosphines and / or Chloride Precursors = ineffective catalysts
Effective Suppression of HomoCoupling
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Mechanism
Reduction to Fe(0)
No significant Fe reduction
F‐ binding to Fe prevents over alkylation and reduction – Precludes Low Valent Mechanisms
C l l ti i di t Role of NHC?Calculations indicate a FeII/FeIV mechanism
Role of NHC?Stabilisation of highly oxidised FeIV?
Substrate Scope
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Conclusions Part 1‐ Cheap metal for a noble task
Fe catalysts can emulate Pd/Ni in numerous cases – with inexpensive ligands
Particularly effective for activation of aryl chlorides and 1o and 2o alkyl C‐X bonds,
Cross Coupling can be very rapid ester groups are now compatiblep g y p g p p
A short Part 2: Where Next for Fe catalysed C‐C Coupling:
1) Stereoselective Coupling (e.g., Knochel ‐ Angew. Chem. Int. Ed. 2011, 50, 3303)
Drawbacks: Can be less general than Pd/Ni catalysis. Still Mechanistic Uncertainty. Seemingly Restricted to Grignard reagents as nucleophiles.
1) Stereoselective Coupling (e.g., Knochel Angew. Chem. Int. Ed. 2011, 50, 3303)
2) Sequential Catalysis : Coupling and then … (e.g., cyclisation + cross coupling)
3) Catalytic C‐H activation: e.g., Direct Arylation without directing groups
2. Sequential (Domino) Catalysis
1) Aryl‐Halide Alkyl Halide One Pot Coupling
0) Cyclisation and Cross coupling
2) Formal sp3‐sp3 coupling. High Yielding with Li[H3BNMe2] as hydride source
Von Wangelin et al., Angew. Chem. Int. Ed. 2009, 48, 607
One‐Pot Coupling
FeCl2 catalyses Grignard formation and cross couplingLow [conc] of Grignard – safety benefits
S. Thomas et al., Chem. Commun., DOI: 10.1039/C1CC14622J
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3. C‐H Activation : with a Directing Group
Applicable to phenylated pyridyls, pyrazoles, pyrimidines, quinolines
D
NN ligands and Mg essential.FeII and FeIII sources are identical
By‐product
D
1 eq. PhMgBr Ph‐D
Mechanism?
O2 not efficient (yields ~ 20 %)
J. AM. CHEM. SOC. 2008, 130, 5858 and 2011, 133, 7672E. Nakamura et al.,
No alkene Reduction, with cyclohexene‐pyridyl no other olefin regioisomers
D
3. Direct Arylation with No Directing Group.
Aiwen Lei et al., Angew. Chem. Int. Ed. 2010, 49, Pg 2004Pg 2004.Fe(III) Precursor
Concurrently: Fe(II)
Charette et al., J. AM. CHEM. SOC. 2010, 132, 1514
40% 86% 85% 79%72 % 69 %
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Definitively not Cu catalysed.
3. C‐H Activation : Direct Arylation
Kinectic Isotope Effect = 1.04
Mechanistic Studies:
Radical Mediated
Charette et al., J. AM. CHEM. SOC. 2010, 132, 1514
Reaction
Use of toluene :
3. C‐H Activation : Direct Arylation
Use of toluene : Regiochemistry (high ortho) supports Homolytic Aromatic Substitution (H.A.S.)
D b kDrawback :1 equivalent of the aryl iodide but100 equivalents of the arene required!
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Proposed HAS Mechanism
Oxidation State of Fe?
Other ObservationstBuOK essential. tBuOHobserved (Sequester HI by‐product) Similar Initiation Step to Mechanism found
in ATRP by Iron (II) catalysts.
Extension to sp3 C‐H Activation : albeit with a Directing Group
FeII/FeIII precursors identical outcomes
J. AM. CHEM. SOC. 2010, 132, 5568E. Nakamura
Indolines, pyrroles, anilines, 2‐pyrrolidones – not viable substrates – Reduction of C‐I to C‐H
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Particularly useful for:1)Coupling of Aryl‐Cl using cheap ligands extremely rapidly (even at low temperatures)2) Different Functional Group Tolerance (Aryl‐I, Aryl Br compatible)3) Radical reactions especially sp3‐C‐X activation and coupling
Certainly a believer in the Iron Age
Reviews in this area:1) Overview of Fe Catalysis (as 2008):
Iron Catalysis in Organic Synthesis (WILEY‐VCH)
2) Fe Cross couplingAcc. Chem. Res. 2008, 41, 1500.ChemSusChem 2009, 2, 396 – 417Ch S R 2008 37 1108
The future ‐ Radical C‐H based functionalisation? Other (less hazardous) Nucleophiles ?
Chem. Soc. Rev., 2008, 37, 1108
3) Alkyl‐Organometallics in Cross CouplingChem. Rev 2011, 111, 1417,
4) Fe Catalysts for Direct C‐H TransformationChem. Rev. 2011, 111, 1293–1314
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1. Coupling with High Diastereo‐ and Enantio‐selectivity
Angew. Chem. Int. Ed. 2011, 50, 3303Knochel et al.,
A Non‐radical Mechanism?